There’s no hard and fast rule for what nominal should be used. For example, some users actually create multiple for different types of sales (services vs goods perhaps). But out of the two you listed, “General Sales” would be more appropriate. Sale of asset would be for anything you purchases and categorised as an asset itself.

For reference, if you go to your chart of accounts and put your cursor over the little question mark, it will explain the nominal code to you:

Personally I’d put the purchase cost as an asset if it’s over a couple of hundred pounds and then the regular bills as an overhead. If the phone itself was free or only cost £50 or something then it can probably just all go as overheads.